Efficient use of explosive energy in blasting to obtain the desired fragmentation and movement of ore and rock continues to be inhibited by restrictions intended to reduce the effect of the blast on nearby structures by minimizing ground vibrations and air blast; delay blasting has been developed to sequence the detonation of the explosives in each hole to increase safety while reducing environmental impact. It has been found however that most of the electric or nonelectric blasting caps and initiators utilize internal pyrotechnic delay elements with the desired delay timing being determined by burn speed of the pyrotechnic composition. As a result, time scatter develops because of variation in burn speed and in the extreme situation, can cause blast holes to detonate out of sequence resulting in significantly increased vibration, poor fragmentation and excessive noise and danger to personnel.
Sequential blasting machines have been developed utilizing electrical circuitry to provide precisely timed initiation pulses to electric blasting caps. The accuracy of the electrical pulses from the sequential blasting machine can be very accurate so as to virtually eliminate timing scatter but electric connections between the blasting cap and the blasting machine must be maintained; broken or shorted connections often lead to undetonated explosives and the hazards resulting therefrom. Moreover, unintended detonations in such electrical systems can be produced by stray electric ground currents, as well as induced currents from magnetic fields from high voltage wires, broadcast stations, radio transmitters and the like.
It has become common to eliminate the hazard attendant to electric blasting caps through utilization of nonelectric transmission lines and nonelectric delay detonators but such systems using, for example, detonating cord on the surface of the blast pattern produces objectionable air blast noise and above-ground noise.
Many efforts have been made by the prior art in endeavoring to solve the multiplicity of noted problems. One such recent prior art attempt to solve the problems is found in PCT Publication No. WO89/01601, the published version of PCT application No. PCT/SE 88/00409 wherein the inventor discloses in very general terms a technique for electronically delayed ignition of an explosive charge based upon input from detonating cord acting upon a piezoelectric element; while that ignition system has a principal purpose to generally eliminate the effect of stray electromagnetic fields and other electrical energy sources, it appreciates the known use of piezoelectric devices with igniters. Piezoelectric driven electrically delayed squib initiators are well-disclosed in U.S. Pat. No. 3,340,811 and a variety of electric delay circuits are to be found in U.S. Pat. Nos. 4,328,751, 4,395,950 and 4,730,556.